Metering device

ABSTRACT

Metering device for metering a readily combustible substance, in particular acrolein acetal, from a source or a container for the substance into a vacuum line ( 38 ) by a pump whose pump power is controllable, characterized in that the pump is driven a compressed air motor ( 28 ) operated displacement pump ( 26 ), and that between the positive displacement pump ( 26 ) and the vacuum line ( 38 ) a pressure holding and regulating valve is arranged whose dimensionless delivery height H/Ho[mWS/1 mWS] with the dimensionless delivery rate Q/Qo[m 3 /h/1 m 3 /h] according to the Formula: 
       { H/Ho}={C 1*( Q/Qo )} 0.333 , where. 
     H=Hydraulic pressure at the inlet of the pressure holding and regulating valve
 
Ho=1 mWS
 
C1=Constant for the pressure holding and regulating valve
 
Q=Hydraulic flow rate
 
Qo=1 m 3 /h.

The invention relates to a metering device for metering a readily combustible substance, in particular acrolein acetal, from a source or a storage container for the substance into a vacuum line by a pump, the pump power of which is controllable.

EP 2 237 373 A1 relates to an apparatus for treating ballast water with acrolein to be connected to a main ballast water line of a ballast water facility, comprising: a reactor apparatus to be supplied with an acrolein derivative, catalyst acid and water for producing an aqueous acrolein solution; a branch line connected to the main ballast water line to the branch off a ballast water partial stream, and a mixing device, which is connected to the branch line and to an acrolein solution supply line from the reactor device and designed for dilution of the aqueous acrolein solution from the reactor device, and a supply device for supplying the aqueous acrolein solution from the mixing device to the main ballast water line. EP 2 237 373 A1 teaches how, on deck of a vessel, an aqueous solution of the biocide acrolein can be produced on demand from acrolein acetal a highly flammable material with a low flash point, plus water and catalyst, which aqueous solution is conducted via a vacuum line into the engine room of the ship in order to disinfect there the ballast water of the ship.

DE 20 2011 032 903 U1 relates to a mixing and metering device for mixing and metering liquid chemicals comprising a circulation pump with a suction nozzle and a discharge nozzle, a conduit coil whose volume content is designed such that the chemicals metered into the device have a sufficient retention time for the chemical reaction, a pitot tube which, while forming a surge point, leads the circular stream exiting from the conduit coil from the outlet of the conduit coil to a metering pipe, which is arranged between the pitot tube and the suction port of the circulation pump and comprises at least two metering valves, as well as a downpipe which is connected to the pitot tube and has a vacuum flange for connection of the mixing and metering device to a vacuum device. It is shown in detail in DE 20 2011 032 903 U1 how, in a continuous apparatus in the form of a loop mixing reactor, the acrolein acetal is reacted with the aqueous catalyst solution, and how these two substances are continuously pumped through two metering valves on this apparatus into a vacuum line.

Metering pumps are usually driven by three-phase motors whose speed is controlled by frequency converters. Frequency converters must be placed relatively close to the three-phase motor, as otherwise there will be control oscillations. In addition, when using the metering pump in potentially explosive environments, such as for example on deck of vessels that transport liquefied gases, the use of electrical operating means is prohibited.

The invention has for its object to provide a metering device for metering a highly flammable substance, in particular acrolein acetal, which can be used safely in hazardous environments.

For this purpose, the metering device according to the invention for metering a readily combustible substance, in particular acrolein acetal, from a source or a storage container for the substance into a vacuum line by a pump whose pump power is controllable, is characterized in that the pump is driven by a compressed air motor displacement pump, and that between the positive displacement pump and the vacuum line a pressure holding and regulating valve is arranged whose dimensionless delivery height H/Ho [mWS/1 mWS] is related to the dimensionless delivery rate Q/Qo [m³/h/1 m³/h] according to the formula:

{H/Ho}={C1*(Q/Qo)}^(0.333), where:

H=Hydraulic pressure at the inlet of the pressure holding and regulating valve

Ho=1 mWS

C1=Constant for the pressure holding and regulating valve Q=Hydraulic flow rate Qo=1 m³/h

By using a compressed air motor instead of an electric motor as the driving means of the metering pump, the device according to DE 20 2011 032 903 U1 can be used in an explosive environment, i.e. on deck of ships carrying liquefied gases. The compressed air line to the compressed air motor can be arbitrarily long, i.e. the compressed air source and its control means can be arranged outside the hazardous area, for example in the engine room of the ship. However, because of the inevitable slip at low compressed air pressures of less than 2 bar, the torque of compressed air motors is non-linear to the compressed air pressure. In terms of control technology, this represents a disadvantage compared to the frequency converters which can be used in three-phase motors. This problem is inventively solved in that a pressure holding and regulating valve is used in the metering line of displacement pump used as metering pump driven by a compressed air motor, where it has surprisingly found that then the number of strokes of the metering pump is proportional to the compressed air pressure on the compressed air motor, so that the metering can reliable be operated with a displacement pump driven by a compressed air motor and the control is simplified.

According to an advantageous embodiment, the metering device according to the invention is characterized in that the pressure holding and regulating valve comprises a cylindrical valve housing having an upper side, in which a central inlet bore is provided with a diameter d1, and with a bottom, in which an inner bore is provided, house diameter d2 is greater than the diameter of the inlet bore and which forms an outlet of the pressure holding and regulating valve, a closure piston with an upper part whose diameter d3 is smaller than the diameter d2 of the inner bore and larger than the diameter d1 of the inlet bore, a freely movable, circular sealing disc made of an elastomer between the closure piston and an inner sealing surface which is formed between the inner bore and the inlet bore on an inner side of the valve housing, and by a compression spring which is supported in the inner bore and presses through the closure piston the sealing washer against the inner sealing surface.

The pressure holding and regulating valve is largely insensitive to the presence of suspended matter in the medium flowing through. The pressure maintenance and control valve according to the invention has a self-cleaning effect, because, since all moving parts can move freely axially and laterally, deposits or accumulations of suspended solids are always flushed out with the fluid. Therefore, the use of the pressure holding and regulating valve according to the invention is particularly advantageous if the pumped medium excretes solids insoluble by polymerization.

According to a further advantageous embodiment, the metering device according to the invention is characterized in that the sealing disc has a diameter d4 which is greater than the diameter d2 of the inlet bore plus the radial extent of the sealing surface, and which is smaller than the diameter d3 of the upper part of the closure piston. These dimensions of the sealing disc ensures in an advantageous manner that the sealing disc covers the inlet bore in any case, regardless of the lateral position of the sealing washer relative to the inlet bore.

According to a further advantageous embodiment, the metering device according to the invention is characterized in that the top and the bottom of the valve housing are formed as a plane sealing surfaces. Thus, the top and the bottom can be used in an advantageous manner as sealing surfaces against connection components when installing the pressure holding and regulating valve.

Further advantages, features and possible applications of the present invention will become apparent from the following description in conjunction with the embodiments illustrated in the drawings.

In the description, the claims and the drawing, the terms and associated reference numerals used in the list of reference numerals below are used.

In the drawings:

FIG. 1 shows a section through a pressure holding and regulating valve which is used in the metering device according to the invention;

FIG. 2 shows a section through an installation of the pressure holding and regulating valve of FIG. 1 in a process plant;

FIG. 3 is a schematic representation of a metering device according to the invention for easily combustible substances, in particular acrolein; and

FIG. 4 is a graph showing the ratio of the compressed air pressure on a compressed air motor of the metering device to the stroke rate of the positive air motor driven by the displacement pump.

First of all, a pressure holding and regulating valve will be described with reference to FIGS. 1 and 2, which is used in the inventive metering device for a vacuum distillation plant.

According to FIG. 1, the pressure holding and regulating valve according to the invention has a cylindrical, metallic valve housing 1 with a top side 2 in which a central inlet bore 3 with a diameter d1 is located. The valve housing 1 comprises an inner bore 5, the diameter d2 of which is greater than the diameter of the inlet bore 3 and which forms an outlet of the pressure holding and regulating valve. The valve housing 1 has an underside 4, which, like the upper side 2, is designed as a flat surface and thus can be used as sealing surfaces against connecting components during installation of the pressure holding and regulating valve.

In the valve housing 1 there is located between the inner bore 5 and the inlet bore 3, a cylindrical, planar, inner sealing surface 6 as a transition between the inner bore 5 and the inlet bore 3. A freely movable, circular sealing disk 7 made of an elastomer, is arranged between a closure piston 8 and the sealing surface 6. As FIG. 1 shows, the diameter d3 of a cylindrical upper part 9 of the closure piston 8 is smaller than the diameter of the inner bore 5 but larger than the diameter of the inlet bore 3.

The sealing disk 7 has a diameter d3 which is greater than the diameter of the inlet bore plus the radial extent of the sealing surface 6. In addition, the diameter d3 of the sealing disk 7 is smaller than the diameter d4 of the upper part 9 of the closure piston 8. The sealing disk 7 thus closes the inlet bore 3 independently of the lateral position of the sealing disk 7 when the closure piston 8 presses the sealing disk 7 against the sealing surface 6. The closure piston 8 has rounded edges 10. A cylindrical lower part 11 of the closure piston 8 has a smaller diameter d4 than the inner diameter of a compression spring 12, which presses the closure piston 8 via the sealing disc 7 against the sealing surface 6.

The compression spring 12 has free passage gaps between the turns. The outer diameter of the compression spring 12 is smaller than the diameter d2 of the inner bore. 5 The compression spring 12 is biased by one or more clamping rings 13. The clamping rings 13 sit with an h6-fit in the inner bore 5, which has a H7-fit. A Seeger-Ring 14, which is clamped in a groove 15, keeps the clamping rings 13 pressed against the compression spring 12 by compression. In the installed state, the compression spring 12 exerts a force on the sealing disk 7 by being pre-stressing by means of the closure piston 8, whereby the inlet bore 3 is closed in a liquid-tight manner.

The compressive force of the compression spring 12 is calculated to the spring constant times compression in mm according to the following formula I:

                                      (Formula  I) ${{Spring}\mspace{14mu} {constant}\mspace{14mu} \left( {N\text{/}{mm}} \right)} = \frac{{material}\mspace{14mu} {constant}\mspace{14mu} \left( {N\text{/}{mm}^{2}} \right)*\left( {{diameter}\mspace{14mu} {of}\mspace{14mu} {spring}\mspace{14mu} {in}\mspace{14mu} {mm}} \right)^{4}}{{Number}\mspace{14mu} {of}\mspace{14mu} {turns}*\left( {{diameter}\mspace{14mu} {of}\mspace{14mu} {spring}\mspace{14mu} {in}\mspace{14mu} {mm}} \right)^{3}}$

As can be seen, there are several free variables to achieve the desired spring tension. The hydraulic pressure of the liquid inflowing at the inlet bore 3, causes the sealing disc 7 to lift off from the plane inner sealing surface 6 against the spring force of the compression spring 12 and that liquid flow entering the inlet bore 3 can flow past the closure piston 8, through a gap 16 between the upper part the closure piston 8 and the inner bore 5 and between the turns of the compression spring 12 into the inner bore 5.

Preferably, the gap 16 has a cross-sectional area which corresponds to the cross-sectional area of the inlet bore 3. During operation of the pressure holding and regulating valve according to the invention, only the spring 12 and the closure piston 8 are moving. The sealing disc 7 therein remains between the closure piston 8 and the sealing surface 6.

The compression spring 12, the closure piston 8 and the sealing disc 7 can move radially in the outlet bore 5 and locate themselves in free play centrically in the inner bore 5, as experiments show. Therefore, the pressure holding and regulating valve according to the invention can advantageously be used on seagoing vessels, where the ordinate axis performs a tumbling motion by the swell.

As shown in FIG. 2, the pressure holding and regulating valve may be installed between two DIN flanges 17, 17′ using flat gaskets 18, 18′. In case the flanges 17, 17′ are parts of two shut-off valves, the pressure-holding and regulating valve according to the invention can be easily removed during operation by locking both valves above and below the pressure holding and regulating valve of FIG. 1 and loosening the bolts 19. The disconnected pressure holding and regulating valve is disassembled by taking off the Seeger-Ring 14, and another compression spring 12 or an additional clamping ring 13 can be replaced in a short time.

The metering device comprises a storage container 20 for a liquid 21 to be transported, for example acrolein acetal. The level of liquid in the storage container 20 may be low, as shown at 22, or high as shown at 23. At the bottom of the storage container 20, a suction line 24 is attached, which leads to a positive displacement pump 26 as a metering pump. The displacement pump 26 is connected via a mechanical coupling 27 with a compressed air motor 28. The compressed air motor 28 is connected to a compressed air line 29 and has an exhaust air pipe 31.

A pressure line 33 leads directly via a flange 17 to the inlet bore 2 of the pressure holding and regulating valve 36, the inner bore 5 of which opens at a flange 17 of a metering line 39. Through an open shut-off valve 40 the medium pumped through the displacement pump 26, acrolein acetal for example, arrives at the vacuum pipe line 59.

The positive displacement pump 26 had a capacity of 27 liters per hour at 128 strokes per minute. In vacuum pipe line 59 there was a vacuum of −9.5 mWS (meters of water). At this flow rate of 27 liters per hour, the pressure gauge in the pressure line 33 indicated a pressure of 25 mWS. The compressed air pressure in the compressed air line 25 was 77 kPa.

As is known, the torque of compressed air motors is non-linear to the compressed air pressure due to the inevitable slip at low pressures of less than 2 bar. Additional measures are therefore required to establish the desired linearity between the compressed air pressure and the stroke rate of the positive displacement pump.

Experiments have shown that, in the pressure holding and regulating valve of FIG. 1, the relationship between the hydrostatic pressure at the inlet bore 3 and the flow rate through the pressure holding and regulating valve, is the ratio of two pressures equal to the third root of the ratio of the respective two volume flows.

If H/Ho [mWS/1 mWS] denotes the hydrostatic pressure in dimensionless form and Q/Qo [m³/h/1 m³/h] the hydraulic flow rate in dimensionless form, the following formula applies to the pressure holding and regulating valve of the invention:

{H/Ho}={C1·(Q/Qo)}^(0.333)  FORMULA II

where: H=Hydraulic pressure

Ho=1 mWS

C1=Constant pressure holding and regulating valve Q=Hydraulic flow rate Qo=1 m³/h

Through the use of this pressure holding and regulating valve, the desired linearity between the compressed air pressure on the compressed air motor 28 and the number of strokes of the positive displacement pump 26 is obtained.

As proof of this, the compressed air motor 28 of the metering device according to FIG. 3 was subjected to different pressures of compressed air and the associated number of strokes of the displacement pump 26 was measured. The results are shown in the graph in FIG. 4. The ordinate denotes the strokes per minute of the positive displacement pump 26, and the abscissa denotes the compressed air pressure on the compressed air motor 28. As can be seen from FIG. 4, the use of the pressure holding and regulating valve 36 of FIG. 1 provides a linear stroke/pressure curve, so that the control of the stroke frequency by means of compressed air control is a fully-fledged, hazard-free replacement for the electric frequency converter.

The following pressure holding and regulating valve according to FIG. 1 was used in the tests, which was defined by the following parameters:

Valve body 1: Material no. 1.2424, outside diameter 27 mm, height=32 mm, Inlet bore 3: Diameter=8 mm Inner bore 5: Diameter=19 mm Closure piston 8: Material no. 1.2424, Diameter upper part 17.2 Sealing washer 7: Viton; Outer diameter 16 mm; Height of plate 2 mm Spring 12: Material no. 1.2330 Outside diameter 17.25 mm, wire thickness 1.25 mm Clamping rings 13: Material no. 1.2424, 3 pieces, height 2 mm Flanges 17: Material no. 1.2771, both sides DN 15

LIST OF REFERENCE SIGNS

-   -   1 Valve Housing     -   2 Upper Side     -   3 Inlet Bore     -   4 Lower Side     -   5 Inner Bore     -   6 Inner Sealing Surface     -   7 Sealing Disc     -   8 Closure Piston     -   9 Upper Part Closure Piston     -   10 Peripheral Edges     -   11 Lower Part Closure Piston     -   12 Compression Spring     -   13 Clamping Ring     -   14 Seeger Ring     -   15 Groove for Seeger Ring     -   16 Annular Gap     -   17 Flange     -   17′ Flange     -   18 Flat Gasket     -   18′ Flat Gasket     -   19 Screw Bolt     -   19′ Screw Bolt     -   20 Reservoir     -   21 Delivery Liquid     -   22 Liquid Level Flow     -   23 Liquid Level High     -   24 Suction Line     -   25 Flow Direction Suction Line     -   26 Displacement Pump     -   27 Mechanical Coupling     -   28 Compressed Air Motor     -   29 Compressed Air Line     -   30 Flow Direction Compressed Air Inlet     -   31 Exhaust Port     -   32 Flow Direction Exhaust Air     -   33 Pressure Line     -   34 Flow Direction Pressure Line     -   35 Pressure Holding and Regulating Valve     -   36 Metering Line     -   37 Shut-off Valve     -   38 Vacuum Pipe 

1. Metering device for metering a readily combustible substance, in particular acrolein acetal, from a source or a storage container for the substance into a vacuum line (38) by a pump whose pump power is controllable, characterized in that the pump by driven by a compressed air motor (28) operated displacement pump (26), and that between the positive displacement pump (26) and the vacuum line (38) a pressure holding and regulating valve is arranged whose dimensionless delivery height H/Ho [mWS/1 mWS] is related with the dimensionless delivery rate Q/Qo[m³/h/1 m³/h] according to the formula: {H/Ho}={C1*(Q/Qo)}^(0.333), where. Hydraulic pressure at the inlet of the pressure holding and regulating valve Ho=1 mWS C1=Constant for the pressure holding and regulating valve Hydraulic flow rate Qo=1 m³/h.
 2. Metering device according to claim 1, characterized in that the pressure holding and regulating valve (35) comprises: a cylindrical valve housing (1) with an upper side (2), in which a central inlet bore (3) is provided with a diameter d1, and with a bottom (4), in which an inner bore (5) is provided, whose diameter d2 is greater than that of the diameter d1 of the inlet bore (3) and which forms an outlet of the pressure holding and regulating valve, a closure piston (8) with an upper part (9), whose diameter d3 is smaller than the diameter d2 of the inner bore (5) and larger than the diameter d1 of the inlet bore (3), a freely movable, circular sealing disc (7) made of an elastomer between the closure piston (8) and an inner sealing surface (6) which is formed between the inner bore (5) and the inlet bore (3) on an inner side of the valve housing (1), and by a compression spring (12), which is supported in the inner bore (5) and presses through the closure piston (8) the sealing disc (7) against the inner sealing surface (6).
 3. Metering device according to claim 2, characterized in that the sealing disc (7) has a diameter d3 which is greater than the diameter d1 of the inlet bore (3) plus the radial extent of the inner sealing surface (6), and which is smaller than the diameter d3 of an upper part (9) of the closure piston (8).
 4. Metering device according to claim 2, characterized in that a gap (16) formed between the upper part (9) of the closure piston (8) and the inner bore (5) has a cross-sectional area corresponding to the cross-sectional area of the inlet bore (3). 